The healing of broken bones, especially for cranofacial fractures, requires realignment of the separated or dislocated bone fragments or segments and subsequent fixation for promoting proper healing of the bones. The presence of relative motion of the bone fragments or segments at a fracture or osteotomy site may result in nonunion between the bone fragments and an extension of the time of fracture healing. It is therefore desirable to accomplish as completely as possible an immobilization of the fracture or osteotomy site. This involves the fixation of affected bone segments.
Much progress has been made in recent years regarding the materials for use in fixation plates, the means for fastening the plates to a patient, and methods and systems for attaching plates to a patient. One common type of device and method of fixing resorbable plates to bone involves insertion of screws through the plate into a hole that must be drilled and tapped in the bone itself. Bone screws typically have a headed portion and an elongated threaded shaft to secure the plate to a bone. One of the problems associated with screws is that they tend to shear and are not self-tapping. Bone fixation plates and screws are typically made of metal or resorbable material.
One example of a metal screw is shown in U.S. Pat. No. 5,578,034 to Estes that describes a method of bone fixation. The method of fixation described in Estes uses a metal screw affixed to the bone through the bore of a fixation plate. The system includes a collar recessed into the underside of the fixation plate which prevents the fixation screw from backing out of the bone once it has been affixed.
Another example of a metal fastener is shown in the Arbegast et al patent, U.S. Pat. No. 5,120,175, which provides a fastener made of a shape memory alloy that will self-deploy when heated above a critical martensitic-austenitic phase transformation point. The shape memory alloys include Raychem K-alloy (Ti-Ni-Cu) and nitinol (Ni-Ti). This fastener, however, is not capable of being used for bone fixation due to the necessity to heat the fastener in the manner described.
Fixation of facial fractures and other reconstructive surgery is desirably accomplished using resorbable material. Resorbable plates can be heated, shaped by cutting, and then contoured to fit over the bone being repaired. A surgeon then drills through an opening in the contoured plate to form a hole in the underlying bone. After tapping threads into the hole drilled in the bone, a resorbable screw is used to secure the resorbable plate to a bone. When the screw is completely seated, if it has a hex-head, the head is shorn off leaving the screw head attached to hold the plate against the bone.
An example of a resorbable screw is shown in U.S. Pat. No. 5,569,250 to Sarver et al. This patent describes a system for securing together adjacent bone fragments. Sarver et al. also describe various designs for resorbable bone fixation plates and screws for use in affixing the plates to bone. The resorbable osteosynthesis plates described in the sarver et al. patent may be used with the fasteners of the present invention. Another example of bone fixation devices which are used to secure a fracture in proximity so that it may be healed using resorbable material is disclosed in U.S. Pat. No. 4,550,449 to Tunc, which describes the use of very high molecular weight polymers of L(-) lactide.
There are several problems with the fixation devices described above. First, these devices and the methods used to affix the plates require a surgeon to both drill and tap threads into the bone. Second, these methods require precision between the depth of the tapped hole and the length of the screw, be it metallic or resorbable. If the tapped hole is shorter than the screw, the screw will not seat completely and thus the plate will not be securely held against the bone being repaired. In this event, the screw must be removed and the drilling/tapping process repeated or the screw may break and need redrilling at a different site. In addition, such breakage may require the plate to be repositioned entirely. Third, the process of screwing, especially a resorbable screw, into a threaded bone opening can result in cross threading and/or breaking of the screw during insertion. Another major problem with metal bone fixation devices is the desirability of removing the devices after the bone has completely healed. The surgical procedure necessary for the removal of metal fasteners results in additional trauma to the patient as well as increased medical costs. In addition, metal devices can interfere with X-ray examination of the fracture area.
In accordance with the foregoing, it is a general object of the present invention is to provide an easy to use and effective fastener blank for attaching an osteosynthesis plate to bone.
Another object of the present invention is to provide a resorbable or radiolucent fastener that once in place is operable to secure a biocompatable plate to one or more bone fragments without the need for tapping.
A further object of the present invention is to provide a fastener for use in securing a fixation plate to bone that will be absorbed by the body.
A still further object of the present invention is to provide a method for securely attaching a fixation plate to a patient.
Yet another object of the present invention is to provide a fastener blank and a method for heating the fastener blank to transform the blank into a fastener securing in place a bone fixation plate.